The present invention relates generally to keyboards and capacitive keys for use in such keyboards, and particularly relates to an improved mechanical design for a capacitive key in which a much less complicated assembly is utilized eliminating the need for a housing or frame for the assembly.
Keyboards such as that to which the present invention relates are widely used in information processing input terminals, for example. U.S. Pat. No. 3,750,113 to Cencel, assigned to the assignee of the present invention, discloses circuit techniques for utilizing capacitive keys of the type described in the present invention. The present invention is directed mainly to the specific assembly of each of such keys. Each key corresponds to a particular piece of data or operation which the operator selects by depressing the key. Typically, a modest pre-load is applied to the key to maintain it in its normal position, thereby reducing the liklihood of accidental actuation and requiring a positive, conscious effort by the operator to depress the key. The key surface touched by the operator is connected to a plunger which extends into the data processing machine for actuating the mechanism of the key therein.
One representative prior art capacitive key arrangement is that described and claimed in U.S. Pat. No. 4,090,229 to Cencel et al. That patent teaches a movable capacitor plate in the form of a single sheet with the plate formed into a spring which is elastically deformed by alteration of the sheet into its deformed condition, with that deformed condition being the "off" position of the key. That particular form of capacitive key requires a housing for supporting the assembly.
With this invention, by contrast, an improved low profile capacitive key is provided as a keyboard switch with a substantially reduced overall height which may be, for example, 0.750 inches (19.05 mm.) for the entire assembly and a length of travel of 0.15 inches (3.81 mm). Because of this, the keyboard switch of the invention is particularly useful for low profile keyboards, as will be appreciated by practitioners-in-the-art. This is achieved, as discussed above, by eliminating the need for any housing for the switch assembly at all and by providing a combination assembly supported directly on the printed circuit board, and wherein the parts are snap-fitted together, eliminating the need for hardware such as screws or nuts for connecting the parts. As will be appreciated, this elimination of a housing and hardware reduces substantially the cost of such an assembly and makes it particularly useful for mass production techniques.
A further feature of the invention is that one plate forming one side of the capacitive key of the invention is developed from the metal laminate formed on the printed circuit board for providing the printed circuit of the keyboard of the invention. It will be appreciated, that by forming the printed circuit and the one plate of the capacitive key of the invention simultaneously, a substantial reduction in cost of the assembly is further achieved.
Furthermore, because of the simplified assembly herein, a plunger part is utilized, connected to the key top, which has a depending plunger switch activation stem of a specific length. Because of the simplified construction of this part, which may be comprised of a molded plastic material, the length of the switch activation stem may be modified in order to modify in a very simple fashion the point of actuation of the switch for the signal change thereof. Thus, by interchanging this one snap-in part of simple signal change point is achieved. Moreover, because the guide stem for the plunger and the switch activation stem thereof have substantial span therebetween, there is good control of the plunger and plunger guide index in the keyboard.
Moreover, because of the arrangement of assembly herein, there is no bounce typical of other switches of this kind when they are turned on. Because of the simple construction of the parts making up the assembly of the switch, in accordance herewith, the only custom part in a keyboard comprised of keys of the invention is the printed circuit board, and the electronics or printed circuits established on that board. It should be appreciated, further, by practitioners-in-the-art that because the electronic components or printed circuits of the switch of the invention including both the movable and the fixed plates thereof are under the printed circuit board, the switching area and the electronic components are protected from liquid spills and plunger dusting.
With the foregoing and additional objects in view, this invention will now be described in more detail, and other objects and advantages thereof will be apparent from the following description, the accompanying drawings and the appended claims.